This invention relates to an improved sealed beam lamp.
A sealed beam lamp having a small-sized halogen bulb in a reflector equipped at its open front side with a lens has recently been developed as a lamp suitable, for example, for a headlight of an automobile. Such sealed beam lamp has the advantage of, for example being higher in light emission efficiency and longer in service life and providing a high output feature.
FIG. 1 is a cross-sectional view showing one form of a sealed beam lamp having a halogen bulb.
A reflector 12 is equipped at its open front side with a lens 14. A halogen bulb 16 has its filament lead lines 18, 20 fixed by ferrules 22, 24 to the reflector 12. A reflective layer 28 such as an aluminum layer is formed on the inner surface of the reflector 12 to reflect a beam of light from the halogen bulb 16. It is required that a beam of light emitted from the sealed beam lamp be focussed onto a predetermined distant spot, that is to say, a beam of light emitted from the filaments of the halogen bulb 16 be externally emitted through the lens 14 after it is once reflected on the reflective layer and converted to a parallel beam of light. Since a scattering light directly externally emitted from the halogen bulb gives a dazzling effect to a driver on the oncoming vehicle it has to be shielded. In general, the filaments of such halogen bulb are arranged axially of the bulb (as often encountered in the European countries) or along a direction of the diameter of the bulb (as often encountered in the U.S.A. and Japan). FIG. 2 is a diagrammatic view showing a conventional halogen bulb structure having filaments along a direction of the diameter of the bulb. In this type of bulb, a heat-resistant light-shielding layer 42 made of, for example, graphite is formed on the upper end surface portion of the bulb to shield a direct external emission of the scattering light. In this arrangement, a beam of light which strikes the inner surface of the upper end portion of the bulb is directed toward, and reflected on, the reflective layer where it is converted into a parallel beam of light so that it can be externally focussed through the lens 14 onto a predetermined distant spot. It is therefore possible to prevent a direct external emission of a scattering light through the lens 14. Since, generally, the ends of the straight filaments 36, 38 of the halogen bulb are located in close proximity to the side wall of the bulb envelope, those side wall portions 44, 46 of the bulb envelope facing the ends of the filaments suffer a greater amount of heat and are heated to a high temperature. For this reason, a harmful gas is produced from the bulb envelope portions 44, 46. Due to the presence of the gas, a halogen cycle becomes somewhat inactivated, leading to an earlier local blackening of the bulb and the attendant reduction of the service life of the halogen bulb.